Japanese Patent Laid-Open No. (“JP”) 2000-156823 discloses an imaging apparatus configured to use an optical characteristic of a part of pixels in an image pickup device in the imaging apparatus differently from other pixels and to implement focus detections based on an output from the part of pixels.
The imaging apparatus disclosed in JP 2000-156823 arranges plural pairs of focus detection pixels in the part of the image pickup device. FIG. 6 shows an illustrative pixel arrangement of an image pickup device that arranges focus detection pixels in some part of rows in a pixel matrix.
In FIG. 6, R, G, and B are normal imaging pixels in which a red filter, a green filter, and a blue filter are respectively arranged. S1 and S2 are first and second focus detection pixels which have different optical characteristics from the imaging pixels.
FIG. 7 shows a structure of the first focus detection pixel S1. In FIG. 7, a micro lens 501 is formed on the light incident side of the first focus detection pixel. 502 is a smoothing layer that has a flat surface configured to form the micro lens 501.
503 is a light shielding layer which includes a stop aperture part that is decentered in one direction with respect to a center O of the photoelectric conversion area 504 in the first focus detection pixel S1.
FIG. 8 shows a structure of the second focus detection pixel S2. In FIG. 8, a micro lens 601 is formed on the light incident side of the second focus detection pixel. 602 is a smoothing layer that has a flat surface configured to form the micro lens 601.
603 is a light shielding layer which has a stop aperture part decentered in an opposite direction from the light shielding layer 503 in the first focusing detection pixel S1 with respect to the center O of the photoelectric conversion area 604 in the second focus detection pixel S2. In other words, the light shielding layers 503 and 603 in the first and second focus detection pixels S1 and S2 include stop aperture parts that are placed symmetrically with respect to the optical axis of each micro lens.
This structure provides an equivalent structure of symmetrical splitting of a pupil in the imaging optical system between the first focus detection pixel S1 and the second focus detection pixel S2.
In FIG. 6, the rows which include the first focus detection pixels S1 and those which include the second focus detection pixels S2 are set such that the two images can be closer to each other as the number of pixels in the image pickup device increases. The rows including the first focus detection pixels S1 and those including the second focus detection pixels S2 have the same outputs (or image signals) when the imaging optical system is in an in-focus state to the object.
On the other hand, when the imaging optical system is in an out-focus state, a phase difference occurs between the image signals derived from the rows including the first focus detection pixels S1 and those including the second focus detection pixels S2. The phase-difference directions are opposite between the front focus state and the back focus state.
FIGS. 9A and 9B show relationships of focusing states and phase differences. In these figures, both focus detection pixels S1 and S2 are moved closer to one another, and referred to as points A and B. The imaging pixels are omitted.
A luminous flux from a specific spot on the object is split into a luminous flux Φ La that is incident upon a focus detection pixel A via the split pupil corresponding to the focus detection pixel A and a luminous flux Φ Lb that is incident upon a focus detection pixel B via the split pupil corresponding to the focus detection pixel B. These two luminous fluxes are incident from the same point on the object, and can reach one point on the image pickup device via the same micro lens, as shown in FIG. 9A, when the imaging optical system is in the in-focus state to the object. Accordingly, the image signals from the rows including the first focus detection pixels A (S1) and those including the second focus detection pixels B (S2) correspond to one another.
However, in an out-focus state by a distance x shown in FIG. 9B, positions which the luminous fluxes Φ La and Φ Lb reach shift by an amount of change in the incident angles of Φ La and Φ Lb to the micro lenses. Consequently, a phase difference occurs between the image signals from the rows including the first focus detection pixels A (S1) and those including the second focus detection pixels B (S2).
With the foregoing in mind, the imaging apparatus disclosed in JP 2000-156823 implements focus control of a phase difference detection method that utilizes the image pickup device.
JP 2001-305415 discloses an imaging apparatus that is suitable for detections of a horizontal line and a vertical line of an object, and configured to implement both focus control of a phase difference detection method that utilizes an output from the image pickup device and focus control of a contrast detection method.
However, the imaging apparatus disclosed in JP 2000-156823 has a difficulty in properly obtaining a phase difference or a defocus amount because two images formed on the first and the second focus detection pixels are asymmetrical to one another as a defocus amount increases.
JP 2001-305415 discloses the imaging apparatus that applies a contrast detection method to focus control when a defocus amount obtained by the phase difference method is less reliable, but this reference is silent about proper use of the focus control of the contrast detection method that utilizes the focus detection pixels and the focus control of the contrast detection method that utilizes pixels other than the focus detection pixels.